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Several Xen developers who currently work for Citrix recently announced they are porting the Xen hypervisor to the ARM processor architecture. The group's work began less than three months ago, but the port is said to already be capable of booting a Linux 3.0-based virtual machine.

The Xen port announcement was made on the Linux Kernel Mailing List by Stefano Stabellini, a senior software engineer on the XenServer team at Citrix, who has been working on Xen technology since 2007. He's joined on the project by several other developers, including fellow Citrix employees Ian Campbell and Tim Deegan, whose history goes back to the early days of Xen with the University of Cambridge and XenSource.

The development team said they wanted to find out how to best support ARM v7+ on Xen, and a few weeks ago they started hacking together a proof-of-concept hypervisor port to ARM's Cortex-A15 reference chip, which uses and requires the ARMv7 virtualization extensions.

In his announcement Stabellini wrote, "The port is based on xen-unstable (HG CS 8d6edc3d26d2) and written from scratch exploiting the latest virtualization, LPAE, GIC and generic timer support in hardware."

While still early in the process, Stabellini said the port is "capable of booting a Linux 3.0-based virtual machine (dom0) up to a shell prompt on an ARM Architecture Envelope Model, configured to emulate an A15-based Versatile Express."

Currently limited to the Cortex-A15, Stabellini says the plan is to support other machines and other chips that are compliant with the ARMv7 architecture and its virtualization extensions. The group is also looking ahead to 64-bit ARM chips, as well as adding support for the ARMv8 architecture, whose specifications were only recently announced a month ago. The group is also working on porting tools and being able to run multiple guests.

But this isn't the only project working on bringing open source virtualization to the ARM architecture.

There is another Xen-based project in the field, called the Xen ARM Project, led by Samsung with newly added help from the Xen.org community. The stated goals of this group are to ensure that Xen ARM can support future ARM-based clients and servers, to merge ARM support into mainline Xen by 2012, and to incubate a diverse and sustainable community around ARM support for Xen.

The Xen ARM Project, led by Sang-bum Suh from Samsung, differs from the Citrix project in that it expands support beyond ARMv7 chips by including support for earlier chip versions (ARMv5 and ARMv6) without virtualization extensions. The project uses Xen Paravirtualization (PV) so that it can support a wider range of chips. The project is also working on other problems such as solving real-time guarantees in a virtualized environment as well as multiprocessor support.

Xen isn't the lone open source hypervisor technology being used in these ARM projects. There's also a group trying to port the open source kernel-based virtual machine (KVM) to the ARM architecture. The KVM for ARM implementation project is currently focused on KVM support for the virtualization extensions of the ARM Cortex-A15. However, the group has already made progress with a slightly modified Linux 2.6.27 and 2.6.29 kernel running KVM compiled for ARMv6 and ARMv7 chips. It is also working on additional performance optimizations, as well as adding support for multicore ARM chips.

While most of the ARM-based chips that enter the market are currently used in smartphones, tablets, and other low-powered, low-performance computing devices, ARM Holdings has other plans for growth. The company is ready to build out its ecosystem and to start expanding into the desktop, server, and HPC markets. In order to do that, virtualization is going to be a key requirement -- making projects like these all the more important.